This application claims the priority of Japanese Patent Applications Nos. 125167/1993, 125168/1993, 125169/1993 and 125170/1993 filed Apr. 28, 1993, which are incorporated herein by reference.
1.Field of the Invention
The present invention relates to a hydrogen-absorbing alloy electrode and, more specifically an improvement in, electrode materials for the purpose of providing a hydrogen-absorbing alloy electrode that hardly undergoes cycle deterioration, and activates in a short time and is readily producible.
1. Description of the Prior Art
In recent years, there have been highlighted metal hydride secondary batteries with a positive electrode of a metal compound such as nickel hydroxide and a negative electrode utilizing a hydrogen-absorbing alloy, which is a new material. This is because they posses higher energy density per unit weight or unit volume, thus realizing higher capacity, compared with other type batteries such as nickel-cadmium secondary batteries.
Ti-Mo-Ni alloys principally containing Ti, Mo and Ni have been attracting attention as negative electrode material, replacing conventional Mm-Ni alloys, for such metal hydride secondary batteries, since they are particularly advantageous to increase battery capacity. Electrodes utilizing this type of alloy are prepared, like those utilizing conventional alloys such as Mm-Ni alloys, by kneading an alloy powder obtained by pulverizing blocks of the alloy with a conductive agent and a binder, applying the obtained mixture on an electrode support and then drying the mixture.
However, the hydrogen-absorbing alloy electrodes using the Ti-Mo-Ni alloy have had the following problems (1) through (3).
(1) Ti-Mo-Ni alloys have been prepared by solidifying an alloy melt at a relatively slow cooling rate of about 10.degree. to 100.degree. C./sec (the cooling rate differs depending on the alloy type and its preparation process). This cooling rate makes the resulting alloy structure nonuniform, and hence electrodes utilizing the alloy tend to undergo, in the course of charge and discharge cycles, surface corrosion to form a dense inactive surface layer, thereby causing cycle deterioration.
(2) When a hydrogen-absorbing alloy is used as an electrode material, it is a common practice to, after fabrication of batteries therefrom, conduct a few times of charge and discharge preliminarily, in order to activate the electrodes by removing oxide film that has formed on the surface during pulverization of alloy blocks and like occasions (hereinafter this operation is referred to as "activation treatment"). Since most Ti-Mo-Ni alloys are covered with a large amount of oxide film, it takes a long time to complete activation treatment on them.
(3) Ti-Mo-Ni alloys are hard and, at the same time, have high ductility, so that such alloy blocks are very difficult to pulverize.
As a result of an intensive study, the present inventors have come to the following findings.
(I) Increasing the cooling rate during solidification can make uniform the resulting alloy structure. Furthermore, forming a layer of hydride on the surface of a hydrogen-absorbing alloy suppresses oxidation of the hydrogen-absorbing alloy.
(II) While increasing the cooling rate during solidification can make uniform the alloy structure and suppresses cycle deterioration in a considerable degree, an alloy that is still more resistant to cycle deterioration is obtained by forming on this occasion, besides a main phase consisting of a Ti-Mo-Ni crystal phase, a subphase of another crystal phase in a specific ratio.
(III) There exists a close relationship between the size and shape of crystallites constituting a main phase in the structure of a Ti-Mo-Ni hydrogen-absorbing alloy and the cycle life of an electrode using the alloy.
The present invention has been completed based on the above findings, and an object of the invention is to provide a hydrogen-absorbing alloy electrode that undergoes little cycle deterioration.